Human CO2 Emissions Blocking California’s Normal Rainfall

STANFORD, California, October 6, 2014 (ENS) – The ‘Ridiculously Resilient Ridge’ formed by human-caused climate change has left California crippled by a withering record drought while diverting life-giving rainstorms to the north, Stanford scientists explain in a new study.

A team led by Stanford climate scientist Noah Diffenbaugh used a unique combination of computer simulations and statistical techniques to show that a “blocking ridge,” a persistent region of high atmospheric pressure over the Pacific Ocean that has diverted storms away from California, was much more likely to form in the presence of modern greenhouse gas concentrations.

“In using these advanced statistical techniques to combine climate observations with model simulations, we’ve been able to better understand the ongoing drought in California,” Diffenbaugh said.

“This isn’t a projection of 100 years in the future,” he said. “This is an event that is more extreme than any in the observed record, and our research suggests that global warming is playing a role right now.”

Scientists agree that the immediate cause of the drought is a particularly stubborn “blocking ridge” over the northeastern Pacific – known as the ridiculously resilient ridge, or Triple R – that prevented winter storms from reaching California during the 2013 and 2014 rainy seasons.

Blocking ridges are regions of high atmospheric pressure that disrupt typical wind patterns in the atmosphere.

The ridiculously resilient ridge now blocking rainfall in California formed off the Pacific coast of Alaska, diverting the storms to the north.

The Ridiculously Resilient Ridge shown on a map of North America (Screengrab from Stanford University video by Kurt Hickman)

“At its peak in January 2014, the Triple R extended from the subtropical Pacific between California and Hawaii to the coast of the Arctic Ocean north of Alaska,” said Daniel Swain, a graduate student in Diffenbaugh’s lab and lead author of the Stanford study.

Swain coined the term “ridiculously resilient ridge” last fall to highlight the unusually persistent nature of the offshore blocking ridge.

Like a large boulder that has tumbled into a narrow stream, the blocking ridge diverted the flow of high-speed air currents known as the jet stream far to the north, causing Pacific storms to bypass California, Oregon and Washington. As a result, rain and snow that would normally fall on the West Coast was instead re-routed to Alaska and as far north as the Arctic Circle.

“Our research finds that extreme atmospheric high pressure in this region, which is strongly linked to unusually low precipitation in California, is much more likely to occur today than prior to the human emission of greenhouse gases that began during the Industrial Revolution in the 1800s,” said Diffenbaugh, an associate professor of environmental Earth system science at Stanford and a senior fellow at the Stanford Woods Institute for the Environment.

Combined with unusually warm temperatures and stagnant air conditions, the lack of rainfall has triggered a dangerous increase in wildfires and incidents of air pollution across the state.

The water shortage will result in direct and indirect agricultural losses of at least $2.2 billion and lead to the loss of more than 17,000 seasonal and part-time jobs this year, experts estimate.

Such impacts prompted California Governor Jerry Brown to declare a drought emergency and the federal government to designate all 58 California counties as “natural disaster areas.”

“Winds respond to the spatial distribution of atmospheric pressure,” said Swain.

“We have seen this amazingly persistent region of high pressure over the northeastern Pacific for many months now, which has substantially altered atmospheric flow and kept California largely dry,” said Swain.

Blocking ridges occur periodically at temperate latitudes, but the ridiculously resilient ridge was exceptional for both its size and longevity. While it dissipated briefly during the summer months of 2013, it returned even stronger by fall 2013 and persisted through much of the winter, which is normally California’s wet season.

Swain says that the precipitation deficit caused by the blocking ridge is so large that it would take”between 10 and 20 major storms, truly heavy precipitation events” between now and next March to bring California back to “where we should be.”

An important question for scientists and decision makers has been whether human-caused climate change has influenced the conditions responsible for California’s drought. Given the important role of the blocking ridge, Diffenbaugh’s team set out to measure the probability of such extreme ridging events.

The team first assessed the rarity of the Triple R in the context of the 20th Century historical record. They found that the combined persistence and intensity of the Triple R in 2013 was unrivaled by any event since 1948, when comprehensive information about the circulation of the atmosphere first became available.

Dry lakebed of Folsom Lake, California, September 26, 2014 (Photo by David Hopkins)

To more directly address the question of whether climate change played a role in the probability of the 2013 event, the team collaborated with Bala Rajaratnam, an assistant professor of statistics and of environmental Earth system science and an affiliated faculty member of the Stanford Woods Institute for the Environment.

Using the Triple R as a benchmark, Rajaratnam and his graduate students compared geopotential heights – an atmospheric property related to pressure – between two sets of climate model experiments.

One set mirrored the present climate, in which the atmosphere is growing increasingly warm due to human emissions of CO2 and other greenhouse gases. In the other set of experiments, greenhouse gases were kept at a level similar to those that existed just prior to the Industrial Revolution.

The researchers found that the extreme geopotential heights associated with the Triple R in 2013 were at least three times as likely to occur in the present climate as in the preindustrial climate.

They also found that such extreme values are consistently tied to unusually low precipitation in California and the formation of atmospheric ridges over the northeastern Pacific.

Rajaratnam said, “We’ve demonstrated with high statistical confidence that the large-scale atmospheric conditions, similar to those associated with the Triple R, are far more likely to occur now than in the climate before we emitted large amounts of greenhouse gases.”

Diffenbaugh’s group was supported in part by a National Science Foundation CAREER Award and by a grant from the National Institutes of Health. Rajaratnam’s group was supported in part by a National Science Foundation CAREER Award, a DARPA Young Faculty CAREER Award, the Air Force Office of Scientific Research and the UPS Fund.